The color matrix and glDrawPixels()

system · February 22, 2002, 11:37am

Hi!

When I draw an image with glDrawPixels() everything works fine. But when I change the contrast or brightness with glScalef() and glTranslatef() on the color matrix, the rendering slows down A LOT! This only happen when I draw with glDrawPixels(). Is it supposed to be like that? Is there any way to make it faster?

Thanks!

chowe6685 · February 22, 2002, 5:05pm

I don’t think the color matrix is hardware accelerated on any commercial cards right now, so it will be slow. You could try emulating the color matrix yourself, though I don’t know if you could make it much faster

system · February 23, 2002, 12:54am

But it is slow only when I draw things with glDrawPixels(). When I draw textured quads or triangles and make changes to the color matrix it’s as fast as if I would’nt modify the color matrix…

Korval · February 23, 2002, 6:05pm

On most consumer cards, glDrawPixels is not a highly acceleratted function. You might be better off uploading the pixels to a texture and drawing that.

system · February 24, 2002, 8:23am

Yeah, I might have to do that, but what if the size of the image isn’t a power of 2?

DFrey · February 24, 2002, 9:59am

Two options, if a texture rectangle extension is available, use it. Otherwise resize the image to the next greatest or equal power of two in each dimension, or just increase the size of the image by simply adding blank space to the right and bottom, and use the correct texture coordinates to address the corner of the texture with the image.